Ammonium heptamolybdate [(NH.sub.4).sub.6 Mo.sub.7 O.sub.24.4H.sub.2 O] sometimes also referred to as ammonium paramolybdate, comprises a valuable source of molybdate ions and is in extensive use as an analytical reagent, is employed for synthesizing molybdenum-containing pigments and catalysts for various liquid and gas phase hydrogenation and desulfurization reactions in petroleum refining operations and coal technology, as well as for the production of a relatively pure grade of molybdenum metal.
A relatively high purity ammonium heptamolybdate or AHM product has heretofore been produced on a commercial scale by dissolving a high purity molybdenum trioxide in ammonium hydroxide to produce a saturated solution having a pH of about 6 to about 6.6, which thereafter is cooled from its saturation temperature of about 55.degree. C to temperatures as low as about 20.degree. C or lower. During the cooling cycle, a precipitation of AHM crystals occurs which are removed such as by centrifugation. The balance of the solution is employed for forming a fresh solution by the addition of further molybdenum trioxide and ammonia to provide the proper pH at 55.degree. C, whereafter the cooling step is again repeated. While the foregoing prior art process is eminently satisfactory for producing an ammonium heptamolybdate product of the requisite purity, certain economic disadvantages are inherent due to the necessity of employing high purity starting materials, such as molybdenum trioxide derived from sublimation or from the calcination of ammonium dimolybdate (ADM) derived from the evaporative crystallization of ammoniacal solutions containing molybdate ions.
The present invention overcomes many of the disadvantages associated with prior art techniques for forming high purity ammonium heptamolybdate as well as ammonium dimolybdate by directly utilizing impure molybdenum oxide concentrates as a starting material, achieving substantial cost savings over techniques heretofore known.